I'm beginning to not like any of them.
I used to have Myspace until around 2006. For one, I couldn't stand it anymore because of its immaturity, and two, I just outgrew it. Since then, I have gotten it 3 times and have quit 3 times, the last time hopefully for good.
I'm beginning to not like Facebook as much anymore because it has lost a lot of quality in the last year. Back when I signed up for it, it was something that you had to wait until you were in college or got invited to it. It made it a lot more special to have one. Now, anyone can sign up for it, and it's becoming more and more like Myspace every day. All it needs is customizable backgrounds and a MP3 player to play when you go to someone's profile, and it'll be Myspace 2.
And Twitter: as of right now, I don't ever plan on getting one, because I wouldn't have enough time to update it, nor would I really want to share with a lot of people what I'm doing all the time.